Hydrogen Adsorption on Nanoporous Biocarbon

Transcription

1 Hydrogen Adsorption on Nanoporous Biocarbon Mikael Wood, Jacob Burress, Cintia Lapilli, Peter Pfeifer, Parag Shah, Galen Suppes University of Missouri-Columbia Phillip Parilla, Anne Dillon National Renewable Energy Laboratory Alliance for Collaborative Research in Alternative Fuel Technology

2 Project Overview As a member of the Alliance for Collaborative Research in Alternative Fuel Technology (ALL-CRAFT) our research group studies the properties of powdered and monolithic nanoporous biocarbon produced from waste corn cob, with the goal of achieving the 2010 DOE gravimetric and volumetric targets for vehicular hydrogen (H 2 ) storage. 200 nm 5.00 μm Pore structure of carbon analyzed via Small Angle X-ray Scattering (SAXS), nitrogen and methane adsorption, and Scanning Electron Microscopy (SEM). ALL-CRAFT is based on the 2002 discovery by Pfeifer et al. of carbons crisscrossed by a nearly space-filling network of channels ~15 Å wide (STM at left).

3 Hydrogen Landscape Our goal: H 2 storage in monolithic, nanoporous carbon at 35 bar Satyapal, 2006 Annual DOE Hydrogen Program Merit Review, Hydrogen Storage Monolithic carbon eliminates dead, interstitial volume. Charging/discharging of the material solely by pressure control eliminates issues of material regeneration and heat management incurred in metal-hydride and chemical hydrogen storage. Low storage pressure enables a flat vehicle tank and minimizes cost of H 2 compression. Our carbon, made from corncob, an abundant, renewable raw material, enables low cost, large scale production of storage material.

4 Data From Nitrogen Adsorption S-33/k BET surface area from nitrogen isotherm ~ 2200 m 2 /g for S-33/k; surface area of latest samples ~ m 2 /g Total pore volume = cc/g Porosity of 0.71 Pore Size Distribution (PSD) from nitrogen shows majority of pore volume is contributed by pores with width < 20 Å. Micropore volume = cc/g, 91% of total pore volume PSD peaked at 6Å and 11Å Peak at 6Å Peak at 11Å S-33/k

5 Pore Size Distribution from Methane Isotherm Shows dominance of nanopores, especially in pores of width 6-15 Å Gives total pore volume of cc/g, porosity of Determined via method from Sosin and Quinn, Carbon (1996) Pore Volume [cc/g] Pore Width Range [nm] S-33/k >50

6 0.500 Pore Volume [cc/g] Methane Nitrogen >50 Pore Width Range [nm] Porosity Total Pore Volume [cc/g] Micropore (pore diameter nm) Volume [cc/g] Mesopore (2 50 nm) Volume [cc/g] Macropore (>50 nm) Volume [cc/g] Average Nanopore Width [Å] Average Nanopore Length [Å] Methane ~11 N/A Nitrogen ~11 N/A SAXS N/A N/A N/A N/A N/A ~4 ~15

7 Small Angle X-Ray Scattering Cartoon of two pore structures that yield similar PSDs, but have vastly different SAXS curves Many methods exist for determining PSDs (N 2, CH 4, Hg, etc.) Small Angle X-Ray Scattering (SAXS) is one of the few methods that allows us to see how the pores are arranged spatially. SAXS provides spatial information over four decades of length (~5Å 50,000 Å)

8 SAXS curve for sample S-33/k 2r Scattered Intensity [1/cm] L () 2 2 I q = r L o Experimental Scattering Calculated Scattering D Surface D 6 () q I q () I q L () I q 1 ql 1.00E E E E E E+01 () I q ( ql ()) J1 qr () qlr sin () cos () 1.0E E E E E+00 ( ) /2 2 2 sin cos * sin = 0 sin () d Scattered Wave Vector [1/Å] Scattering from a cylinder, with finite thickness. 1.00E-01 Scattering in the limit L>>r

9 Definitions of Uptake Values Pore Absolute Adsorbed Gas Stored Gas Adsorbed Film Bulk (non-adsorbed) Gas Gibb s s Excess Adsorbed Gas m = m m ( m m ) + V absolute BulkGas adsorbed Chamber, Sample, Gas Chamber, Gas Chamber, Sample Chamber 1 AdsorbedFilm BulkGas Skeletal m = m m ( m m ) BulkGas Stored Chamber, Sample, Gas Chamber, Gas Chamber, Sample Chamber 1 Piece Excess madsorbed = mchamber, Sample, Gas mchamber, Gas ( mchamber, Sample mchamber ) 1 BulkGas Skeletal

10 Hydrogen Excess Adsorption Isotherm 60 Excess Mass Adsorbed for Mass Sample [g/kg] S-33/k 298 K Adsorption 298 K Desorption 77 K Adsorption 77 K Desorption Pressure [bar] Hiden IGA-001; measurements courtesy Hiden Isochema, Ltd., Warrington, UK

11 Absolute Adsorption Isotherm Mass Adsorbed for Mass Sample [g/kg] S-33/k 298 K Adsorption 298 K Desorption 77 K Adsorption 77 K Desorption Pressure [bar] Langmuir fits drop below experimental data at low pressures, which is consistent with hypothesis that higher binding energy sites are getting filled first and then the lower binding energy sites 298 K isotherm: asymptotic mass for mass of 27 g/kg and a Langmuir parameter of b= bar K isotherm: asymptotic mass for mass of 72 g/kg and a Langmuir of b= bar -1

12 Storage Isotherm 70 Stored Mass Adsorbed for Mass Sample [g/kg] S-33/k 298 K Adsorption 298 K Desorption 77 K Adsorption 77 K Desorption Pressure [bar]

13 Hydrogen Uptake S-33/k Hiden (extrapolated) S-33/k ALL- CRAFT S-33/k Parilla AX-21 (*) MOF-177 (**) 77K, 47 bar 7.9 mass% mass% ~8 mass% 5.1 mass% ~10 mass% 300K, 47 bar 1.2 mass% mass% mass%.6 mass% 2.4 mass% The values in the table reflect amount stored (both adsorbed and non-adsorbed gas). Our storage values have been independently verified and compare well with the best performing carbons in the literature. (*) - E. Poirier, R. Chahine, P. Bénard, D. Cossement, L. Lafi, E. Mélançon, T.K. Bose, and S. Désilets, Storage of hydrogen on single-walled carbon nanotubes and other carbon structures. Appl. Phys. A 78, (2004). (**) (a) O.M. Yaghi, Hydrogen storage in metal-organic frameworks DOE Hydrogen Program Review, ST22. (b) NSF News Release (3/9/06): New crystal sponge triples hydrogen storage.

14 Conclusions and Outlook Uptake values cross validated (reliable). Our carbon sample S-33/k with amount stored of ~1.5 and ~8 mass% at 300 K and 77 K (47 bar) compares favorably with the best performing materials in the literature. Thus far, all samples have been optimized for methane storage. We are optimistic that their performance will be greatly improved once they are optimized for H 2 storage. Our carbon is cost effective. Highest performing carbons have a correlated pore structure. SAXS data will aid in future design of higher performing pore architectures.

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16 5.00 μm 200 nm 5.00 μm 100 nm

17 Density Functional Theory Pore Volume Distribution Non-Local Density Functional Theory (NLDFT) for slit-shaped pores was used. Relationship between this theory and the experimental data is given by the generalized adsorption isotherm (GAI) W MAX N P N P, W = f ( W) dw P P 0 0 W MIN where N(P/P0) is the experimental adsorption isotherm data, W is the pore width, N(P/P0,W) is the isotherm on a single pore of width W, and f(w) is the pore size distribution function.

18 Brunauer-Emmett-Teller Surface Area Most widely used method for determination of surface area of solids BET Formula given by: m C P P 0 = mmono 1 P 1 ( C 1) P P + 0 P 0 where m is the mass of gas adsorbed at a relative pressure (P/P 0 ) (P 0 taken as the saturation pressure of adsorptive gas), m mono is the mass of adsorbate constituting a monolayer of surface coverage, and C is the BET constant.

19 BET Theory Cont. BET equation in linear form: m P P 0 1 C 1 = + P P 0 1 P Cmmono Cmmono P 0 intercept = slope = 1 Cm C 1 Cm mono mono = m mono 1 slope + intercept Total Surface Area = S = Total mono A CrossSection N A is Avogadro s number, A CrossSection is the cross-sectional area of the gas molecule, and M is the molecular mass of the gas. A CrossSection for Nitrogen is 16.2 Å 2 /molecule m N A M

20 SAXS curve for sample S-33/k D Surface D 6 () q I q 2.3 2r L 2 2 () = o I q r L () I q 2 L () I q 1 ql () I q ( ql ()) J1 qr () qlr sin () cos () ( ) /2 2 2 sin cos * sin = 0 sin () d Scattering from a cylinder, with finite thickness. Scattering in the limit L>>r